The continuous circulation of white immune cells throughout the body is our defense against disease caused by bacteria, viruses, harmful chemicals, as well as our built-in, 24×7 surveillance system against the development of cancer. A healthy body sees between 4,000 and 11,000 white cells per microliter of circulating blood, but this concentration increases in response to a threat. Psychological stress has an immuno-suppressive effect by reducing the white cell count and thus the body’s ability to fight diseases ranging from the common cold to cancer.
How Stress Affects the Immune System
The direct communication between the sympathetic nervous system (SNS) and the immune system consists of adrenergic projections and sympathetic nerve terminals that are found in many organs of the body, such as the spleen. An acute SNS activation by a stressor causes the immediate release of catecholamines from nerve endings, initiating the automatic arousal that takes place during the stress reaction. This neuroendocrine response to stressors also increases the levels of glucocorticoids (primarily cortisol) in circulation, which are steroid hormones that in addition to rapidly mobilizing the body against the threat also have an effect on the immune system.
Acute vs. Chronic Stress
Stressors, depending on their nature and duration, modulate the functions of the immune system by influencing the number of white cells circulating in the bloodstream. The effects of a brief, acute stressor (e.g., a sudden noise) on white cell circulation are short-lived and subside when the stressor passes. There are longer lasting effects on white cell circulation when the stressor is prolonged and severe (e.g., a relationship problem), as in chronic stress.
Regardless of its origin, psychological stress always leads to a change in white cell count at varying degrees depending on the type and duration of the stressor. Current research shows that longer-lasting stressors cause a reduction of immune function and increase our vulnerability to disease. Numerous studies document the immune system suppression caused by severe stressors such as marital strife, bereavement, long-term caregiving, living in unfavorable conditions, and by the psychological reaction to environmental disasters such as floods, earthquakes, fires, and hurricanes.
Stress, Immune System and Cancer
According to the National Cancer Institute’s current information on the possible association between stress and cancer, at least three areas of investigation are being explored: stress effects on virus-related cancer, stress effects on cell processes, and stress effects on tumor growth and spread.
Virus-related Tumors. An indirect relationship between certain types of virus-related tumors (Kaposi sarcoma, Burkitt lymphoma, cancer of the liver) and stress has been established. The indirectness results from the fact that some cancers are triggered by a process that involves certain precursor infections (such as herpes and hepatitis) that are known to be exacerbated by stress and a weakened immune system.
Cell Processes. The body’s natural neuroendocrine response has been shown to alter important cell processes that protect against the formation of cancer, such as DNA repair and the regulation of cell growth. Age-related deficits in protein synthesis and the responsiveness of cells to stress, decreased cell-cell communication, and inefficient signal transduction may render old cells less able to withstand stress (genotoxic stress).
Tumor Growth and Spread. The precise biological mechanisms underlying the influence of stress on the growth and spread of cancer are not yet well understood, but a link between the effects of stress on the immune system and the growth of some tumors has been documented. A recent study at the University of Texas Anderson Cancer Center in Houston indicates that stress hormones, especially norepinephrine and epinephrine, can contribute to tumor progression in patients with ovarian cancer.
Other Factors in the Effects of Stress on the Immune System
There are many factors that can exacerbate the negative influence of stress on the immune system. Age, nutrition, gender, ethnicity, and psychosocial characteristics of the individual can affect white cell circulation in response to stressors. Depression, lack of social support, or a hostile personality can cause altered immune cell responses to acute stress. Among the protective factors, physical fitness appears to be a very important positive mediator of white cell activity in the presence of psychological stressors.